The cost of a private beach starts at €15 for a lounge chair and umbrella and runs to €30 per person, more for a sunbed. Since it is forbidden to bring your own food to a private beach, add in the cost of a meal. A main course at a private beach-restaurant starts at €14 for a pizza, pasta or large salad and runs to €30 for a grilled fish.
At first glance, it's easy and obvious to say that going to a free public beach is cheaper than laying out money for a managed private beach. If you live on the Cote d'Azur that's certainly true because you already have a beach towel, mat, umbrella, shoes and plenty of other equipment to make your day at the beach comfortable. When you come as a visitor however, your beach experience can require a substantial cash outlay. Hotels do not provide beach towels or beach umbrellas. Buying a towel and umbrella in central Nice can easily run about €25. By contrast, the cheapest private beach in Nice is Cocoon which costs €15 for a lounge chair and umbrella with a towel for €6. Other beaches cost more of course but you get the idea.
To be truly comfortable on a public beach, you would want a rubber or bamboo mat under your beach towel. Count on at least €12. Obviously, that can't compare to a lounge chair topped with a mattress. To get from the water to your towel, you'll at least need a pair of flip-flops (€5) but even better are rubber water shoes (€10). On a private beach, bambou mats run down to the water's edge, making the extra footware unnecessary.
And what about food and drink? You can picnic on a public beach for much less than a beach-restaurant. The pan bagnat sandwich that you can buy for €5 in a sandwich shop costs €14 in a beach restaurant and that's usually the cheapest item on the menu. You do not save money however, by ordering a meal in a cafe/restaurant along the Promenade des Anglais as their prices are roughly the same as their counterparts on the beach.
The bottom line? For just a day at the beach as part of a visit-the-French Riviera -experience, it's more comfortable and not much more expensive to spend the day at a private beach. If you'll be hanging at the beach for two or more days, invest in the proper acoutrements and you'll have a fine time at a public beach.